1. Field of the Invention
The invention relates to a particle-optical apparatus, an electron microscopy system and an electron lithography system.
2. Brief Description of Related Art
Particle-optical apparatuses operating with beams of charged particles are used, among others, in the manufacture of miniaturized devices, such as semiconductors, both for the manufacture itself and the examination of the structures which are manufactured. Examples of this are electron microscopy systems and electron lithography systems. However, the invention is not limited to these systems, but relates to particle-optical apparatuses in general which manipulate beams of charged particles of all kind, such as electrons and ions.
From the prior art there are known, for example, scanning electron microscopes (SEM) which make use of lenses for focusing an electron beam in an object plane. The lenses provide focusing electric fields, focusing magnetic fields or superpositions of focusing electric and focusing magnetic fields. In order to provide the fields for a charged-particle beam, the lens must comprise a physical assembly surrounding the beam and providing the corresponding field source members, such as electrodes or/and magnetic pole pieces. This imposes limitations on the configuration of the fields manipulating the beam. The limitations are due to limits in respect of the mechanical precision in the manufacture of the source members as well as in respect of the mechanically feasible geometries of the field source members.
Further, a particle-optical apparatus is known from the art which is referred to hereinafter as a “comb lens” and which is described in further detail hereinafter with reference to FIGS. 1 and 2a-2c. 
The particle-optical apparatus 1 comprises three electrode arrangements superimposed in the z-direction, namely a slit electrode 3 at the bottom comprising an aperture 5 elongated in the x-direction, a slit electrode 7 at the top comprising an aperture 9 likewise elongated in the x-direction, as well as a comb electrode arrangement 11 disposed between the two slit electrodes 3 and 7. The comb electrode 11 comprises two rows of source members for electric fields, namely finger electrodes 13, which are disposed on both sides of a central longitudinal axis 15 of the comb electrode 11. The central axis extends in the x-direction. A space in the xy-plane between the two rows of finger electrodes 13 may be regarded as an aperture of the comb electrode 11.
Electric potentials are supplied to the two slit electrodes 3 and 7 as well as to the finger electrodes 13 by a controller, not shown in FIG. 1, so that adjustable electric fields can be generated between the electrodes 3, 7 and 13. These fields act on a beam of electrically charged particles which is oriented transversely to the xy-plane and traverses the apertures of the electrodes 7, 11 and 5. If an electric potential is applied to the slit electrodes 3 or 7 which is different from the potential of the beam of charged particles in the plane of the respective slit electrode 3, 7, the effect of the slit electrodes 3 and 7 respectively exerted on the beam is that of a cylinder lens. A configuration of the electric field lines as it is generated by such a slit electrode 3, 7 is schematically shown in FIG. 2a. 
A potential pattern can be applied to the finger electrodes 13 of the comb electrode 11 such that a quadrupole-like electric field is generated in the aperture of the slit electrode 11. A configuration of field lines of such quadrupole field is schematically shown in FIG. 2b. The quadrupole field has an axis of symmetry 17 which extends in the z-direction and intersects the longitudinal axis 15 of the comb electrode 11.
A beam of electrically negatively charged particles entering such quadrupole field is focused in the x-direction and defocused in the y-direction.
Thus, when a beam enters the apparatus 1 along the axis of symmetry 17 of the quadrupole field, it is subjected as a whole to the effects of the cylinder lens fields provided by the slit electrodes 3 and 7 according to FIG. 2a as well as of the quadrupole field provided by the comb electrode 11 according to FIG. 2b. The beam is thus subjected to a superposition of the field configurations shown in FIGS. 2a and 2b. When the strengths of the cylinder lens fields and the quadrupole field are appropriately adjusted to each other, the effect exerted on the beam is the same as that produced by a round lens field, the field lines of which are schematically shown in FIG. 2c. 
It is thus possible to focus a beam of charged particles by means of the apparatus 1 if appropriate voltages are applied to the electrodes 3, 7 and 13. Further, the beam may be scanned to any position along the longitudinal axis 15 of the comb lens, and it is possible to energize the comb electrode such that the axis of symmetry of the quadrupole field coincides with the beam. The beam will then experience the focusing effect of the comb lens irrespective of its position along the longitudinal axis.
As the quadrupole field to be formed for generating the round lens effect is generated in this apparatus by a limited number of discrete finger electrodes, the generated field will deviate from an ideal quadrupole field as a result of a discretization error. This constitutes a limitation for the comb lens in practical use. Moreover, such comb lens can only be manufactured with sufficient precision with a very high expenditure in mechanics.